Well not exactly unexpected, my MC18 with suspect squish and pistons seized today at the track after a few good races.

Great fun it was too, went like a rocket. I'll whip the heads off sometime this week and see how it is - a friend was racing it in the final race of the day at the time and managed to avoid a highsiding a dogleg bend at the end of the front straight and clutched to a halt. A good save

Barrels on that thing were far from pristine anyway, hopefully the bottom ends avoided damage.

hi mine done the same today, what is the likley hood of bottom end damage,

Unusual for a seizure to affect a bottom-end... unless you totally destroy the piston and get debris amongst the big-ends or something.

Usually it's only the top that will go.
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And more often the bottom cylinder in my experiences !

OK so I gently rocked the flywheel with a socket and it freed up, so I whipped the heads off and we've certainly got something different happening between the two cylinders!

Strangly, or not both pistons are free to move in the cylinders and don't appear damaged at all. I'm 90% certain this was heat related rather than a mechanical failure. Now whether it was due to a lean mix or leaking coolant (it had a pinhole leak but was topped prior to the race and had completed two prior 8 lap races) or failed water pump (using a non gycol coolant for racing that was of cooking oil consistancy!?).

I had an external temp gauge on the top cylinder since the dash one was unhooked. Pitty I didn't have a baseline having never run it before, after my first few races the outside of the top barrel was 66 celcius or so - pretty hot. I know my RG150 runs 40-55c usually (i'd seen it hit 70+ before) so I wasn't too concerned.


I'll add pictures, hopefully you guys can give me some advice. One cyl wasn't firing all the time at idle, but kicked in no worries with a few revs and on the track it was pulling plenty well.

Running 33:1 premix fully syth castrol TTS with 98RON gas, standard jetting airbox minus filter but not drilled or anything. No sign of piston contact (scratch on the carbon of the top cyl pic is just from a head stud lightly touching it when removing)


Bottom head


Top head


Bottom piston


Top piston


Top cylinder plug


Bottom cylinder plug


So the top looks to me like its been running and the bottom very little, although completely differently from the sparkplug electrode color? wonder if one of the carbs got blocked. Top cylinder plug ok or a bit lean?

Is 66c too hot etc,


What say you wizards The ring on the bottom piston is from before head machining, it wasn't contacting - and you cant see it on the top piston from the carbon

I wouldn't like to be the crank that was attached to those pistons when they were touching the heads. Has no one heard of squish or tunring a motor over once its rebuilt?

From the pictures you can tell, even if one cylinder was running very rich that teh squish on each were very different to each other.
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If I have to take the carbs off once more...

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Last edited by DifferentStrokes on Mon Sep 28, 2009 6:26 pm; edited 1 time in total

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Last edited by DifferentStrokes on Mon Sep 28, 2009 6:26 pm; edited 1 time in total

top piston looks like there is a bit missing!!!

That motor needs a serious coat of looking at!

You can tell nothing from those plugs other than the thread on one is more oily that the other! You don't read the tip of the insulator to give an indication of jetting, and you need to see the whole electrode to make any assumptions regarding ignition.

It doesn't look well though, that's for sure, and rectifying it is well beyond the scope of a rejet! That motor needs to come apart completely else the next time it goes you could end up on your ass!

I'd start by sourcing a set of standard heads, measuring the heights of the barrels, and inspecting the crank for (previous) damage.

Not sure about the "coolant" you are using either.
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Andy.
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DifferentStrokes wrote:+1 with S-RC45

The head setup seems more aggressive than the ignition to me.

Sure it was spoted, + maybe alittle al transfer (good save)

The bit you've marked isn't infact damaged its from a droplet of coolant darkening the carbon.

When the bike had the heads machined yeah crank and rods was checked for play and wasn't sloppy or notchy.

That coolant is "Sikolene Fuchs CCA Pro Ultra"



thanks for the advice guys, I'll see how I go. Barrels in NZ aren't really available even second hand and the cost of them is more than entire bike cost me.

And that's a consistency similar to light cooking oil? It's just a new one on me.

I use Rockoil Kool or water... both have the same consistency.
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Yeah mate it's a new one to me as well!

I was pretty hesitant putting it into the cooling system, can be mixed with 5% water or something but its pretty strange stuff.

In my RG150 it took a good 15c off the barrel temps when racing compared with the old stuff I had in it so I was pretty pleased.

Thats if the pumps are ok to push that sludge..

I suppose the squish must've been a fair bit different between the two cylinders, I'd never measured it before until I put the heads back on last time and was shown how with a bit of solder on the top cylinder from memory.

I'll see if the guy who machined them can help me out (also works and preps RS125's and 150 2-strokes). It was his son who was on it when it seized, an experienced rider thankfully who managed to keep it upright and probably saved it from more damage.

Well i'm a bit closer to realizing my overheating hypothesis. I had a few minutes spare just before so I whipped off the water pump cover ..





Strange/worst bit is there is no sign of the blade in the housing so somebody has known about it before, removed it and ignored it .. Temp gauges on both my cylinders from now on. Next step is pulling the carbs and cleaning them out and checking the jetting on them, which i've done before and am 99% sure is factory.


Replacing that pump, split cases job?

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Last edited by DifferentStrokes on Mon Sep 28, 2009 6:16 pm; edited 1 time in total